A line of men in black rain boots push trash carts through the alleys of Lahore, Pakistan. They stop at an open sewer along a neighborhood street and start to pull up shoes, bricks, plates and any other trash that might block the flow of wastewater.

Standing water is a prime breeding ground for mosquitoes. And the local government in Lahore is on a focused mission: Stop the spread of dengue fever by mosquitoes.

Two years ago, an estimated 20,000 people in and around the city of Lahore contracted the deadly tropical disease. This year, the region has recorded just a few dozen cases of dengue fever, which usually involves a high fever, horrible headache, and severe bone and joint pain.

What triggered the sharp decline in dengue cases? Fortuitous weather patterns may have helped to keep the mosquito population low. But many leaders also credit a mobile phone app and the public health campaign that uses it.

"We pull up the trash, put it in the basket, tie up the bag and take it away," says sanitation worker Tanvir Channa. He says that he doesn't often think about his role in combating a deadly epidemic. "Whatever I do, it's just to provide for my kids," the thin 30-year-old says.

To make sure workers like Channa don't skip out on their tasks and allow the dengue mosquitoes to breed, they're followed by an investigator who uses a smartphone to their progress. In this case, it's a tall man in plaid shirt named Mohammad Saleem Taqi.

"I open this application, called Clean Lahore, to enter a field activity," he says. "I take pictures before and after the work is done, enable location services to map this spot, and then send it on to my supervisors."

"Of course it seemed strange at first," Channa says, of having his picture taken on the job. But now he believes the monitoring campaign is to his benefit because the photos show supervisors that he's on the job and can't be marked absent.

Across town from the sewer, men with the fishery department tip a bucket of water into a small neighborhood pond. Dozens of tiny tilapia fish swim into the pond. These fish have a taste for mosquito larvae and naturally curb the mosquito population.

As the two men work, an inspector snaps a photo of them with the Clean Lahore app.

The app is the brainchild of Umar Saif, a Cambridge-educated computer scientist, who now manages part of the anti-dengue campaign.

"So let me tell you the story from the top," Saif says, settling into a couch in the center of his office in a Lahore high-rise building.

For him, the story begins in the summer of 2011. "What happened is, Punjab was hit with one of the worst dengue epidemics anywhere in the world."

Government officials realized they would need to work harder and smarter to prevent another epidemic. That's why they turned to Saif. He developed a smartphone app to track all efforts to prevent the disease. And the idea has contributed to the city's striking success against dengue.

"If Punjab averted another epidemic in 2012, then it didn't happen by accident," Saif says. "There were 67,000 different prevention activities [that] were performed and were photo-logged by the smartphones."

Other public health researchers have suggested that the decline in dengue cases might be because of environmental factors, at least in part. But still, the government's prevention campaign has been widely lauded.

One reason for the accolades is that Saif took the mobile campaign one step further: He built a Google map that correlates the locations of dengue cases and hot spots for mosquito larvae. "So there's a clear pattern of disease outbreak that corresponds to reports of positive dengue larvae," he says.

With these visuals, Saif and his team could zero in on problem regions in the province and predict future outbreaks.

The mobile phone campaign also helps to stop another issue that plagues Pakistan: entrenched public sector corruption.

"You have people who have not done maybe for decades work as well as they were supposed to do," Saif says. "So the government needs to therefore now use technology in innovative ways to monitor its functions."

"This is quite remarkable," says Patty Mechael, about the anti-dengue campaign. She studies how mobile technology is helping public health at the nonprofit mHealth Alliance.

When it comes to tracking infectious diseases with cellphones, Mechael says, the possibilities are endless. "It's really up to the health sector to imagine what it needs, and then think about where mobile technology can actually play a role in solving some of those problems."